Interesting, I'd like to know the answer too. Are you using a Yahoo app? I just have GMail and when it downloads to the phone, that's it, it's on the phone. If I delete an email online before turning the phone on it doesn't get downloaded to the phone. It'd be great if would work like your Yahoo mail.

I switched my user agent to iPhone3, went to m.google.com/sync, disabled javascript, but that is as far as I get. Not sure what to do next. I don't have a PB and there is no other devices listed on the page.

have you checked your BIS settings and checked the sync deleted items by chance? Maybe that could be it? Mine delete.

Mine is like the OP's. If I delete a mail in Yahoo on the web, it disappears from my BB within minutes. If I delete a Gmail on the web, it stays on my BB forever, unless I delete it there manually. The option to sync deleted items has always been checked, but as far as I can tell, it's only a one-way sync with deleted mail.

Mine is like the OP's. If I delete a mail in Yahoo on the web, it disappears from my BB within minutes. If I delete a Gmail on the web, it stays on my BB forever, unless I delete it there manually. The option to sync deleted items has always been checked, but as far as I can tell, it's only a one-way sync with deleted mail.

Exactly, and it sucks having to delete emails twice. Makes me wish I never started using gmail to begin with. Wonder what is different between yahoo and gmail to cause this?

It is a lack of Gmail features from google. they do not support Gmail sync for blackberrys.
this feature is only available for iPhone, Nokia, and Windows (outside of android of course).
Maybe something to do with having to go through BIS???